Long-term exposure to biodegradable polylactic acid microplastics promotes Microcystis aeruginosa proliferation

Biodegradable plastics, such as polylactic acid (PLA), are widely considered as sustainable alternatives to petroleum-based polymers; however, PLA generates polylactic acid microplastics (PLA-MPs), which raises ecological concerns in freshwater ecosystems. In this study, we investigated the responses of Microcystis aeruginosa to PLA-MPs (1, 10, and 50 mg/L) across a 45-day co-culture experiment. We found the PLA-MPs initially inhibited algal growth (days 18, 27) through mechanical damage and shading effect, as indicated by the activation of oxidative stress system, heterogeneous aggregation of algae cells and PLA-MPs, and down-regulated expression of light reaction in photosynthesis. However, during stabilization (days 36 and 45), PLA-MPs promoted growth in a dose-dependent manner, with Chl-a content in the 1, 10, and 50 mg/L PLA-MPs added groups reaching 1.12, 1.36, and 1.44 times that of the control, respectively. And the particle size of algal cell aggregates increased upon PLA-MPs exposure. In response to the dual negative and positive effects of PLA-MPs during 45 days of co-incubation, M. aeruginosa exhibited dynamic biological responses. This demonstrated that the intracellular MDA content, SOD activity, CA content, MC-LR content, as well as extracellular MC-LR content and EPS (polysaccharides and proteins) content exhibited increases before day 27 followed by a decrease after day 27, which correspond to the two distinct phases of M. aeruginosa growth inhibition and promotion. Especially, the expression of proteins involved light reaction in photosynthesis was inhibited in the on day 27, while it recovered and was promoted on day 45. Our results confirm that PLA-MPs have a long-term effect on M. aeruginosa growth, providing fundamental theoretical guidance for investigating the ecological and environmental impacts of PLA-MPs in aquatic ecosystems.